CMS, Java

Apache Sling Resource Filter – Alternate to JCR Queries

Default AEM Query Engine works lazy, that means it doesn’t load all results directly when doing the request.1000 queries per request will never perform well if when rendering a product page he wanted to references the assets associated to it. This speaks of how expensive JCR Queries could instead be replaced with targeted content structure traversals to allow finding relevant content without searching the whole AEM repository. Unfortunately, from a developer perspective, this means writing a fair amount of boilerplate code to traverse the AEM repository and then identify the relevant Resources.

Fortunately, a new API was just released by the Apache Sling team. This API adds support for  using Lambda expressions to filter a stream of Resources from the Sling repository.

This new Resource Filter API allows AEM / Sling developers to be significantly more succinct and readable and how they perform common repository traversals. It’s not a pure replacement of JCR queries or or the traditional method of traversing the repository, but for simple content structures you can do a lot more with a lot less code.

Resource Filter Example

Here’s an example of how one could extract a stream of resources by a page’s component type under a cq:Page:

The equivalent pre-Stream code is quite a bit more verbose, even in this simple example:

In this example, we are just writing the resource to a JSON Array using the javax.json API, but once you have a stream of Resources, you can use any Lambda function to then process or further filter the stream.

Adding the Resource Filter API in your Project

Adding the Resource Filter API into your project takes two steps. First add the dependency into your project’s POM:



More interesting things are Resource Predicate Service,that allows you to convert a string that defines a simple matching requirements into a Predicate<Resource> for use with the Collections and the Streams Java API. In addition it also allows you to add parameters to the underlying context that the script will use.

Resource Stream

ResourceStream is a general utility to provide a Stream<Resource> which traverses a resource and it’s subtree. The implementation takes a Predicate<Resource> object as part of the stream creation to define a branch selector that controls which children of a resource are followed.

In addition there is a getChildren(Predicate) method which returns a filtered list of children of the given resource.

Resource Filter Stream

ResourceFilterStream combines the ResourceStream functionality with the ResourcePredicates service to provide an ability to define a Stream<Resource> that follows specific child pages and looks for specific Resources as defined by the resources filter script. The ResourceStreamFilter is access by adaption.

Similar to indexing in a query there are strategies that you can do within a tree traversal so that traversals can be done in an efficient manner across a large number of resources. The following strategies will assist in traversal optimization.

Limit traversal paths

In a naive implementation of a tree traversal the traversal occurs across all nodes in the tree regardless of the ability of the tree structure to support the nodes that are being looked for. An example of this is a tree of Page resources that have have a child node of jcr:content which contains a subtree of data to define the page structure. If the jcr:content node is not capable of having a child resource of type Page and the goal of the traversal is to identify Page resources that match a specific criteria then the traversal of the jcr:content node can not lead to additional matches. Using this knowledge of the resource structure, you can improve performance by adding a branch selector that prevents the traversal from proceeding down a non productive path

Limit memory consumption

The instantiation of a Resource object from the underlying ResourceResolver is a non trivial consumption of memory. When the focus of a tree traversal is obtaining information from thousands of Resources, an effective method is to extract the information as part of the stream processing or utilizing the forEach method of the ResourceStream object which allows the resource to be garbage collected in an efficient manner.

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